Nothing is foolproof (better fools are created every day!).
http://www.sitepoint.com/forums/apache-configuration-199/how-do-i-block-visitors-who-use-proxy-servers-766303.html shows Q&A about proxy servers, however, it's possible to block a number of IP addresses (of known proxy servers). The main problem arises if a truly anonymous proxy server is used as these do not identify themselves OR that the request has been relayed.
IMHO, the easiest way for you is to try a "fuzzy" match for usernames and passwords that are too similar to be a coincidence, time of day of visits by these "members," etc.
Of course, a sign-up with e-mail confirmation within a short time span makes it difficult for bot makers to respond but adds a bit of a task for a real person (IMHO, well worth the small effort).